A patterned beam of EUV light can be used to expose a resist coated substrate, such as a silicon wafer, to produce extremely small features in the substrate. Extreme ultraviolet light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5-100 nm. One particular wavelength of interest for photolithography occurs at 13.5 nm and efforts are currently underway to produce light in the range of 13.5 nm+/−2% which is commonly referred to as “in band EUV” for 13.5 nm systems.
Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not necessarily limited to xenon, lithium and tin.
In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a source material, for example in the form of a droplet, stream or wire, with a laser beam. In another method, often termed discharge produced plasma (“DPP”), the required plasma can be generated by positioning source material having an EUV emission line between a pair of electrodes and causing an electrical discharge to occur between the electrodes.
As indicated above, one technique to produce EUV light involves irradiating a source material. In this regard, CO2 lasers outputting light at infra-red wavelengths, i.e., wavelengths in the range of about 9 μm to 11 μm, may present certain advantages as a so-called ‘drive’ laser irradiating a source material in an LPP process. This may be especially true for certain source materials, for example, source materials containing tin. One advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.
For LPP and DPP processes, the plasma is typically produced in a sealed vessel, such as a vacuum chamber, and monitored using various types of metrology equipment. In addition to generating in-band EUV radiation, these plasma processes also typically generate undesirable by-products. The by-products can include out-of-band radiation, high energy source material ions, low energy source material ions, excited source material atoms, and thermal source material atoms produced by source material evaporation or by thermalizing source material ions in a buffer gas. The by-products can also include source material in the form of clusters and microdroplets of varying size and which exit the irradiation site at varying speeds. The clusters and microdroplets can deposit directly onto an optic or ‘reflect’ from the chamber walls or other structures in the chamber and deposit on an optic.
Unfortunately, some or all of the plasma formation by-products described above can potentially damage or reduce the operational efficiency of one or more optics in the chamber including, but not necessarily limited to, collector mirrors capable of EUV reflection at normal and near normal incidence and/or grazing incidence, the surfaces of metrology detectors and filters, windows used to image the plasma formation process, and the laser input window/lens. The plasma formation by-products may be damaging to the optical elements in a number of ways, including heating them beyond their service temperature, coating them with deposits which reduce light transmission or reflection, and may cause undesirable corrosion or erosion. In addition, plasma formation by-products may penetrate or diffuse into an optic causing damage to the optic's structural integrity or a reduction in optical performance. Deposits can include, but are not necessarily limited to, the source material, contaminates present in the source material, oxides of the source material/contaminates, and organics from chamber components that have not been properly cleaned or that outgas in the vacuum environment.
In some instances, it may be desirable to introduce one or more cleaning species into the chamber to remove deposits from an optic in-situ (i.e., while the optic is in the chamber and under vacuum conditions). In one mechanism, one or more cleaning species can combine with a deposit and form a volatile substance which can then be pumped from the vacuum chamber. For these processes to be efficient, a sufficient quantity of the cleaning species needs to be present at the surface of the deposit and it is generally desirable to quickly pump the volatile substance from the chamber before the volatile substance can decompose and re-deposit debris. Volatile substances in the chamber can also undesirably absorb EUV light. This absorption can reduce EUV light source output and/or efficiency.
During operation, the output beam from an EUV light source may be used by a lithography exposure tool such as a stepper or scanner. These exposure tools may first homogenize the beam from the light source and then impart the beam with a pattern in the beam's cross-section, using, for example, a reflective mask. The patterned beam can then be projected onto a portion of a resist-coated wafer. Once a first portion of the resist-coated wafer (often referred to as an exposure field) has been illuminated, the wafer, the mask or both may be moved to irradiate a second exposure field, and so on, until irradiation of the resist-coated wafer is complete. During this process, the scanner typically requires a so-called burst of pulses from the light source for each exposure field. For example, a typical burst period may last for a period of about 0.5 seconds and include about 20,000 EUV light pulses at a pulse repetition rate of about 40 kHz. The length of the burst period, number of pulses and repetition rate may be selected based on EUV output pulse energy, and to the accumulated energy, or dose, specified for an exposure field. In some cases, pulse energy and/or repetition rate may change during a burst period and/or the burst may include one or more non-output periods.
In this process, sequential bursts may be temporally separated by an intervening period. During some intervening periods, which may last for about a fraction of a second, the exposure tool prepares to irradiate the next exposure field and does not need light from the light source. Longer intervening periods may occur when the exposure tool changes wafers. An even longer intervening period may occur when the exposure tool swaps out a so-called “boat” or cassette which holds a number of wafers, performs metrology, performs one or more maintenance functions, or performs some other scheduled or unscheduled process. Generally, during these intervening periods, EUV light is not required by the exposure tool, and, as a consequence, one, some, or all of these intervening periods may represent an opportunity to remove deposits from one or more optics in the light source chamber.
With the above in mind, Applicants disclose Systems and Methods for Optics Cleaning in an EUV Light Source.